For aerating waste water in basins or tanks, various different types of aeration systems can be used. Frequently, however, the aeration system employed is one by means of which the aeration is effected with a uniform distribution of the air bubbles in a region relatively close to the location of the aerator and in the immediate vicinity of the floor of the basin but not over the entire expanse of the basin floor. Such systems are, for example, ones which utilize pressurized aerators, jet nozzle aerators, immersion aerators, static mixers, and the like. In most cases where such an aerator is used, therefore, only a portion of the basin or tank is intensively aerated.
Merely by way of example, one type of immersion aerator which can be advantageously used in waste water aeration is disclosed in U.S. Pat. No. 3,891,729 and its progeny. Such an aerator includes, in essence, an immersion motor-driven rotor or turbine rotatable in the center of a guide ring, which turbine aspirates air automatically or under a minimal precompression and centrifuges it in a finely divided state and together with indrawn liquid approximately radially outwardly of the guide ring.
Since the generation of very fine air bubbles, which are a prerequisite for achieving a high SOTR when the level of the liquid in the basin is at a relatively low point, for example, at a height of between about 2 m and 4 m above the basin floor, is difficult to achieve by means of such aeration systems, the industry has frequently tried to avoid this problem by opting for large liquid heights, in which case, however, in order to avoid a too high energy consumption, the air must be fed into the aerator under a certain degree of precompression. The large liquid heights enable a better OTE to be achieved by virtue of the longer residence time of the rising air bubbles in the liquid, which in turn leads to a larger SOTR. It is, nevertheless, not always possible to install very deep aeration basins. Quite to the contrary, frequently it is feasible only to install aeration basins with a limited depth.
In this context, a peculiar phenomenon has been observed. On the one hand, an immersion aerator installed in a tank 3.8 m in diameter was tested under a liquid height of 4 m, and a very good SOTR characterized by an OTE of approximately 35% and an SAE of 2 kg O.sub.2 /kWh resulted. Thereafter, the same immersion aerator was tested in a waste water basin 10.times.10 m in size and at a liquid height of 4 m, and it was determined that the OTE dropped to about 20% and the SAE dropped to about 1.2 kg O.sub.2 /kWh. The initially inexplicable cause of this phenomenon became clear, however, after many tests.
The immersion aerator installed in the 3.8 m diameter tank was able to aerate the tank uniformly over its entire cross-section, but could not perform correspondingly in the large basin. Basically, each quantity of rising air performs work through its expansion, which work manifests itself in the elevation of a certain quantity of liquid. In the 3.8 m diameter tank, the elevated liquid level remains stationary, in other words, a balance is established between the constantly rising liquid and the liquid simultaneously descending between the air bubbles. The air bubbles rise approximately at a velocity of 0.2 m/s through a body of liquid 4 m high and thus have a residence time of approximately 20 seconds until they reach the surface of the liquid.
In the larger basin, which cannot be totally aerated, the relationships are fundamentally different. The air bubbles rise initially uniformly through a generally columnar region above the centrifugation zone of the submersible aerator, which region, depending on the size of the aerator, is approximately 4 m in diameter. The work generated by the expansion of the air bubbles, as previously mentioned, drives the liquid upwardly. As the level of the liquid above this region is elevated somewhat, the elevated liquid flows at first radially outwardly and then, after a certain outward flow, begins to flow back downwardly until, when near the floor of the basin, it flows back toward the center of the aeration region. As a result of this flow, the descending liquid throttles the air emission from the aerator. This causes the rising air, and with it the liquid, to be confined to a somewhat smaller cross-section, although the quantity of displaced liquid remains the same since it depends only on the work output of the rising quantity of air.
In such a case, the velocity of upward flow of the liquid attains values which lie between 0.2 and 0.5 m/s. The gas bubbles, however, rise about 0.2 m/s faster than the liquid and thus reach the upper surface of the liquid in a very short time, for example, within 6 to 10 seconds. That means that the residence time of the air bubbles in the liquid becomes as small as it would be if the height of the body of liquid in a small vessel would be only 1.2 to 2 m. As a result, the OTE and therewith the SOTR decreases correspondingly. The cause of this can thus be seen to reside in the liquid circulation which is created, which is also known as the "airlift effect".